Particulate resins needing uniformity, such as a particulate toner for electrophotography, a particulate spacer, a colored particulate for electron paper and a particulate for pharmaceutical medicine bearer are used in various applications. As a method of preparing a uniform particulate, a method of inducing a reaction in a liquid to form a particulate resin having a uniform particle diameter such as a soap free polymerization method is known. The soap free polymerization method has advantages, e.g., a particulate resin having a small particle diameter can easily be prepared, the particulate resin has a sharp particle diameter distribution, and the particulate resin nearly has a spherical shape. However, the particulate resin is subjected to de-solvent in a solvent which is typically water in the soap free polymerization method, and the particulate resin is not efficiently prepared. Further, the soap free polymerization method needs a long time in its polymerization process, separates the particulate resin from the solvent after solidified, and needs to repeat washing and drying then. Therefore, the method has a problem of needing much time, water and energy.
In order to solve this problem, the present inventor disclosed a toner preparation method by spray granulation in Japanese published unexamined application No. 2008-286947.
Specifically, in a droplet spray unit spraying a toner component liquid as a toner material, a thin film on which plural nozzles are formed is oscillated by an electromechanical transformation element as an oscillator to periodically oscillate the thin film up and down. Thus, a pressure in a liquid chamber partially formed by the thin film is periodically changed to discharge a droplet from the nozzle in a gas-phase space below the nozzle. The discharged toner droplet naturally falls in the same direction in the gas-phase space to form a line of the toner droplet. The toner droplet discharged in the gas-phase space is transformed to be spherical by a difference in surface tension between the liquid phase of the toner composition and the gas phase, and then dried and solidified to form a toner.
Japanese Patent No. 3786034 discloses a method of reserving a toner material liquid at a head, discharging the liquid by a pressure pulse, and drying and solidifying the liquid to prepare a particulate toner.
However, when the pressure pulse is driven, a gas dissolved in the liquid reserved at the head generates a microscopic gas bubble causing a problem of defective discharge.
In order to solve this problem, a method of using a deaerator removing the gas dissolved in the liquid to prevent the gas bubble from generating is known.
Further, when the liquid pressure to the head is too high or too low, the liquid exudes from a discharge hole of the head or air mixes in, which causes problem of unstable discharge of the droplet.
In order to solve this problem, Japanese published unexamined application No. 2011-98302 discloses a method of locating a tank reserving a liquid fed to an inkjet head and a pressure sensor monitoring a pressure of the liquid in the tank, and discharging the liquid until the pressure sensor indicates a desired pressure to control the pressure to the head.
However, when continuously discharging droplets and consuming a large amount of the liquid as the toner preparation apparatus by the spray granulation method, the method disclosed in Japanese published unexamined application No. 2011-98302 discharges a large amount of the liquid to control the liquid pressure to the head, resulting in increase of environmental load and cost.
Because of these reasons, a need exist for an environmental load-reducing and low-cost particulate production method properly controlling the discharge pressure to stabilize discharging.